When a plurality of communication devices performs data transmission sharing the same medium such as wireless or high-speed PLC, it is necessary to perform control such that two or more communication devices do not perform data transmission to the same medium at the same timing. This is because, when two or more communication devices perform data transmission to the shared medium at the same timing, the transmitted data collide on the transmission path, the data are not separated in the reception devices, and thus communication fails.
Conversely, when the transmission access to the shared medium is extremely restricted, no communication device obtains access right to the shared medium even if there is a communication device having data and desiring the transmission access to the shared medium, and a state of disusing the shared medium continues, which is not preferable from the viewpoint of transmission efficiency. As a technique of controlling the access of this communication device to the shared medium, there is medium access control (MAC).
In addition, there is “low power data communication system/wide band mobile communication access system (HiSWANa)” (ARIB STD-T70), standardized as a radio communication standard including such a medium access control method, and the like.
Hereinafter, the medium access control employed in the HiSWANa standard will be briefly described. A system configuration of HiSWANa is based on a radio communication system including an access point (hereinafter, referred to as “AP”) and a mobile device (hereinafter, referred to as “MT”), and employs a TDMA (Time Division Multiple Access) method in concentrated control of the AP. The TDMA method is a method of alternately sharing one frequency with a plurality of transmitters (AP and MT) for a short time each. In the HiSWANa standard, Broadcast CHannel (hereinafter, referred to as “BCH”), Frame CHannel (hereinafter, referred to as “FCH”), and Access Feedback CHannel (hereinafter, referred to as “ACH”), for network management are transmitted from the AP to each MT. Among them, the BCH is used to take time synchronization between AP and MT, and is used to perform network association request with respect to the AP after performing detection and reception of the BCH to establish time synchronization when a new MT is associated to the network. The MT completing the network association receives the FCH including schedule information of the data transmission timing and the data reception timing, measures timing according to band assignment for transmission and reception, and performs transmission and reception of data. Since the MT for which the access to the shared medium is allowed at a specific timing is limited to one device by the schedule information described in the FCH, data collision does not occur in the radio communication path that is the shared medium, medium access control is realized, and it is possible to secure stable communication.
As the TDMA method employed in the HiSWANa, the AP schedules the assignment of the transmission band with respect to the device (also including the AP itself) having the transmission data, reflects it to the FCH, and transmits it for an MT. Accordingly, the MT having the data to be transmitted requests the assignment of the transmission band to the AP, and thus the band can be assigned. A message for requesting the band by the MT is a band assignment request message, and the AP receives the message to check a transmission buffer state of each MT to schedule a band of a certain period so as to occupy a specific MT, and reflects it to the FCH that is the schedule information.
As described above, each MT has to transmit the band assignment request message to the AP using several bands in the same manner as transmission of user data. However, since the band scheduled for the FCH is scheduled for the first time after the band assignment request message is transmitted to the AP, the first band assignment request message cannot be transmitted. For this reason, in the HiSWANa, a Random CHannel (hereinafter, referred to as “RCH”) band is separately provided. In the RCH band, since the access to the shared medium is allowed so as not to be controlled to the schedule of the AP by the FCH, the MT can transmit the band assignment request message to the AP at an arbitrary timing. However, since the RCH period is out of a target of the access control by the AP as described above, there is a case where the plurality of MTs transmits the band assignment request message at the same timing. In this case, collision of the transmission data may occur, and the data reception of the AP on the reception side may fail. For this reason, the MT transmitting the RCH receives the ACH of the next cycle, and has to detect whether the band assignment request message transmitted to the AP in the RCH band of the previous period is received by the AP or collision occurs on the transmission path and the reception in the AP fails.
The MT detecting that the RCH transmission of the previous cycle fails by the reception of the ACH stops the access to the shared medium at a random period, and then performs retransmission of the band assignment request message based on the RCH. This is a system for making the timing of performing the retransmission between the devices different to avoid the band assignment request message based on the RCH transmitted at the next time from colliding again between the colliding MTs, which is referred to as back-off control.
As for the demand assignment type of the access right (time slot) to the shared medium by the band assignment request from individual MTs such as HiSWANa described above, a new proposal is made from the viewpoint of improvement of utilization efficiency (transmission efficiency) of the shared medium. In PLT 1, collision occurrence frequency of RCH used for transmission of a band assignment request message for original control and ASsociation Control CHannel (ASCH) transmitted when a new MT is associated is monitored. When the collision occurrence frequency is equal to or less than a predetermined value, data transmission itself, and not a message for control, is made possible from an MT to an AP in the RCH to improve the line utilization efficiency.